Soil-repellent and abrasion-resistant monofilaments and methods of making and using same

ABSTRACT

The process of making the soil-repellent and abrasion-resistant monofilament includes transesterifying dimethylterephthalate with ethylene glycol; adding to the resulting reaction mixture from 0.15 to 5 percent by weight of a polydialkylsiloxane of formula I: 
     
         --CO--R.sub.1 -- Si(R.sub.2).sub.2 --O!.sub.n --Si(R.sub.2).sub.2 --R.sub.1 
    
      --CO--                                                    (I), 
     wherein n is the number of repeated --Si(R 2 ) 2  --O-- groups and R 1  or R 2  represent the same or different radicals with 2 to 20 carbon atoms; then polycondensing to form a linear silicone-modified polyethylene terephthalate in which the polydialkyl siloxane of formula I is inserted into a polyethylene terephthalate polymer chain thereof; and extruding the linear silicone-modified polyethylene terephthalate to form a monofilament consisting of the silicone-modified polyethylene terephthalate having a silicon content of 0.05 to 2.0% by weight. The monofilament product has a cross-section with a diameter of from 0.1 to 1.0 mm, a homogeneous composition over the entire cross-section, a tenacity of at least 36 cN/tex, an elongation of less than 44% based on an unstretched portion and a soiling index of less than 10 after five soiling cycles.

BACKGROUND OF THE INVENTION

The invention relates to a process for producing soil-repellent andabrasion-resistant monofilaments of a diameter of between 0.1 to 1.0 mmfrom linear polyethylene terephthalate/polydialkyl siloxane forproducing screen fabrics, as to the monofilaments produced by theprocess and to methods of using the monofilaments.

"Soil-repellent" properties for screens of paper-making machines aregenerally understood to mean wet soiling properties, which alsopresupposes hydrolysis resistance. The soil repellence is intended to beused for extending the cleaning cycles of a screen, wherein cleaning isperformed under a high-pressure water stream and, if required, byrubbing with a brush. Soil-repellent monofilaments are worked intoindustrial fabrics which are employed in the wet portion, in thepressing as well as the drying portion of a paper-making machine. Suchscreens are subjected to moisture and temperatures far above 100° C., aswell as increased mechanical stress during cleaning. Pure polyestermonofilaments have disadvantages based on their tendency to soil, theirsusceptibility to hydrolysis and their relative brittleness. Animprovement in their hydrolysis resistance was achieved by addingadditives in the form of stabilizers. An increase in soil repellency wasattempted, besides the addition of additives, also by suitable finishingtreatments of the filament and aftertreatment of the fabric.

"Abrasion-resistant" monofilaments are understood to be thosemonofilaments with which no actual weight loss could be noted duringfibrillation testing as well as only little or no fibrillations.

It was found that co-polyesters modified with siloxanes were excellentlysuited for the above-mentioned field of use, in spite of their lowhydrolysis resistance, because their surface properties are easilymodified to provide improved sliding properties and smaller coefficientsof friction, by which the brittleness is clearly reduced and soilrepellency considerably improved. The formed surfaces can be easilytreated again to improve their chemical properties, such as hydrolysisresistance.

Polyester/polydimethyl siloxanes, which are available either ingranulate form or are modified by additives directly prior to extrusion,have proven to be especially suitable. Polyoxysilanes of the type whichconstitute modified polyesters and are provided for use in accordancewith the invention are known from U.S. Pat. No. 5,123,392. The knownchemically modified polymers are used for producing hydrophilic staplefibers, but are also said to be usable for fabrics made of filamentyarns. The hydrophilic properties make possible the rapid absorption ofmoisture and are therefore mainly used in a mixture with cotton or wool.In their present form yarns having the hydrophilic properties mentioned,however, are unsuited for paper-making machines or fishing lines.

Silicone-modified polyesters which are suitable for producing fibers arealso known from EP-A-0 269 023. In the course of producing the polymer,the silicon units are formed in a polyester matrix. During extrusion inair and drawing, these silicon units migrate into the outer envelope ofthe filament. The surface obtained acts like a hydrophobic film. Thechemical composition is not homogeneous in cross section.

SUMMARY OF THE INVENTION

It is the object of the invention to make available a monofilament whichis simultaneously soil-repellent and abrasion-resistant and homogeneousin cross section, meets the great demands made on an industrial fabricin respect to its chemical, soil-repelling, mechanical andabrasion-resistant properties and finds its use in the paper industry inparticular.

The object of the invention is attained by means of the invention inthat 0.15 to 5.0 weight-% of polydialkyl siloxane are inserted into thechain of the polymer by co-condensation, i.e. in the course of thepoly-condensation.

The tenacity of the monofilaments is greater than 36 cN/tex with anelongation of <44% referred to the unstretched filament, wherein aparticularly good soil repellency and little fibrillation occur asessential characteristic in a surprising manner. The soil repellency hasbeen determined to be a soiling index of <10 following fivesoiling/cleansing cycles, compared with >14 for a polyester filamentwithout polydimethyl siloxane.

It is practical to use a polyester consisting of at least 85 weight-% ofpolyethylene terephthalate and a polydialkyl siloxane, wherein thepolyethylene terephthalate/polydialkyl siloxane in the finishedmonofilament contains 0.05 to 2 weight-%, preferably 0.1 to 1.0 weight-%of silicon referred to the polymer, and the homogeneous mixture isextruded into a monofilament. It should be stressed as a particularadvantage that the produced monofilament has a homogeneous compositionover the entire cross section.

A polyethylene terephthalate to be considered is one which has anintrinsic viscosity of >0.60 dl/g.

Difunctional polydiorgano siloxanes of the general formula: ##STR1## arepreferably considered as copolymers, wherein n is the number of repeated--Si(R₂)₂ --O-- groups and R1, R2 represent equal or different radicalswith 2 to 20 carbon atoms. Polydialkylsiloxanes especially polydimethylsiloxanes (PDMS) with R2=CH₃ have proven themselves to be particularlysuitable.

The silicon content in the monofilament is 0.05 to 2.0 weight-%, inparticular 0.1 to 2 weight-%, preferably 0.1 to 1.0 weight-%.

The use of the soil-repellent monofilaments in accordance with theinvention, having a diameter of 0.1 to 1.0 mm, in particular 0.2 to 0.6mm, is particularly suited to the production of screen fabrics forpaper-making machines.

The invention will be described in detail by means of an example.

Exemplary Embodiment

The production of the polymer takes place in a known manner bytransesterification of dimethyl terephthalate and ethylene glycol andthe addition of 3.9 weight-% of polydimethyl siloxane (PDMS) andmanganese acetate in stirred autoclaves at 160 to 230° C. The generatedmethanol is quantitatively distilled off. The subsequentpoly-condensation is performed over 200 min in a temperature rangebetween 245° to 285° C. and with the use of an antimony catalyst,wherein condensation is performed in a first step in a progressivevacuum (1000-<50 bar) over 40 minutes and in a second step in a highvacuum (>1 mbar).

The polyethylene terephthalate/polydimethyl siloxane (PET/PDMS) ismelted at a temperature of 290° C. Extrusion takes place by means of aspinneret with 13 capillaries and a capillary diameter of 1.8 mm at apolymer flow rate of 486 g/min. The extruded monofilament are cooled ina water bath at 70° C. After cooling the monofilaments are spin-preparedand stretched to a residual elongation of approximately >44%, relaxedand wound at a speed of 100 m/min. The resulting fiber has a soilingcoefficient of less than 10 (<10) after five treatments, with a 500%stretching has a tenacity (Ft) of 36.1 cN/tex at an elongation (Dt) of38.8% referred to the unstretched monofilament.

However, in place of the PET/PDMS as the polymer it is also possible tostart with pure, commercially available polyester(polyethylene-terephthalate) and to add polydimethyl siloxane (PDMS)upstream of the extruder by means of a metering device.

The following method was developed for the determination of thesoil-repellent effect of monofilaments for screen fabrics forpaper-making machines:

Principle of the Testing Method (Soiling Test)

Rust-resistant small steel plates are exactly wound with three layers ofthe monofil. After rinsing out the spinning preparation and calorimetricmeasurements, the top of the sample is soiled eight times in accordancewith a fixed method by "transfer printing" with wet newsprint in anironing press at 130° C. Following calorimetric measuring, cleansing,drying and another calorimetric measurement are performed in a specificmanner. This soiling/cleansing process is performed five times in a rowfor the simultaneous determination of the "soil release" properties. Ateach stage the DL* (D65)-values are indicated as quantitative results,with the cartels before they were first soiled respectively used asreferences. DL* (D65) corresponds to the brightness difference betweenthe sample and the reference in daylight. To exclude the effects ofcolor differences in the samples, the brightness decrease in percent isindicated, which is defined as

    AI=- DL*!/L*.sub.0 ×100

and is called the soiling index.

Principle of the Test Method (Fibrillation Test)

To determine the fibrillation of monofilaments for screen fabrics forpaper-making machines, a bundle of monofilaments of the same kind isbeaten against a standardized solid body for 10 minutes at a frequencyof 2000 to 3200 rpm.

The results of the fibrillation test are combined below, wherein thegrading has the following meaning:

Grading of the Fibrillation

0.0-0.5: no or very weak fibrillation

0.5-1.5: weak fibrillation

1.5-3.5: moderate to strong fibrillation

3.5-4.5: very strong fibrillation

>4.5: very great damage

The Fibrillation test results are collected in the following table.

                  TABLE I                                                         ______________________________________                                        FIBRILLATION TEST RESULTS                                                              Grade at Beat Frequency  rpm!                                        Example    2000    2500       3000 3200                                       ______________________________________                                        1          0.0     0.1        0.5  2.5                                        2          0.5     1.0        0.9  4.5                                        3          0.4     1.1        1.5  5.0                                        4          0.2     0.8        1.0  5.0                                        ______________________________________                                    

Example 1 shows the result with the monofilament in accordance with theinvention, wherein no fibrillation at all occurs at 2000 beats perminute, while at 3200 beats a fibrillation of less than half that of theprior art occurs. Example 2 shows the result with a polyester made"soil-repellent" by means of a fluoropolymer, and the Examples 3 and 4,standard polyesters.

Furthermore, the monofilaments produced in accordance with the inventionfrom PET/PDMS are less rigid than known monofilaments.

BRIEF DESCRIPTION OF THE DRAWING

The test results for the exemplary monofilament of the present inventionwill now be illustrated in more detail with reference to theaccompanying drawing in which:

FIG. 1 is a graphical illustration showing tear force versus elongationresults for the monofilament according to the invention;

FIG. 2 is a graphical illustration showing soiling test results; and

FIG. 3 are photographs illustrating the fibrillation test results.

From the force/elongation diagrams in FIG. 1 it can be seen that in asurprising manner the tenacity of the monofilament in accordance withthe invention has not been reduced by copolymerization.

The values of the soiling indices represented in FIG. 2 show the effectsof successive "soiling/cleansing" cycles. Mean values of two samplesafter soiling and cleansing are respectively represented. The curve 1represents the monofilament fiber in accordance with the invention.Curve 2 represents a comparison fiber of polyester in accordance withthe prior art, i.e. without the polydimethyl siloxane component.Following (five) 5 soiling/cleansing cycles, indices below 10 areattained with the monofilament in accordance with the invention.

FIG. 3 shows in the form of photographs the results of a fibrillationtest. FIG. 3a shows a polyester with a fluoro-polymer; FIG. 3b themonofilament in accordance with the invention. It can be seen from thisthat the monofilament in accordance with the invention showsconsiderably less fibrillation. Under the same conditions, a sample madeof polyethylene terephthalate in accordance with the prior art evenshows a large weight reduction.

In the soiling test the monofilament fiber in accordance with theinvention results in an improvement by at least a factor of 1.5 inrespect to the prior art. The soil repellency of the fibers modifiedwith siloxanes shows a clear improvement in comparison with the standardand approximately corresponds to that of the fluorine-containing specialfibers which are questionable for reasons of environmental load. Theabrasion resistance of the fibers is clearly increased in respect to theprior art. The monofilaments in accordance with the invention arepreferably suited for use in paper-making machine fabrics.

While the invention has been illustrated and described as being embodiedin methods of making and using soil-repellent and abrasion-resistantmonofilaments, it is not intended to be limited to the details shown,since various modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A soil-repellent andabrasion-resistant monofilament consisting of a linear silicone-modifiedpolyethylene terephthalate having a silicon content of from 0.05 to 2.0%by weight based on the monofilament in which 0.15 to 5.0% by weight of apolydialkyl siloxane of formula I:

    --CO--R.sub.1 -- Si(R.sub.2).sub.2 --O!.sub.n --Si(R.sub.2).sub.2 --R.sub.1 --CO--                                                    (I),

wherein n is the number of repeated --Si(R₂)₂ --O-- groups and R₁ or R₂represent equal or different radicals with 2 to 20 carbon atoms, isinserted into a polyethylene terephthalate polymer chain thereof;wherein said monofilament has a cross-section with a diameter of from0.1 to 1.0 mm, a homogeneous composition over said cross-section, atenacity of at least 36 cN/tex, an elongation of less than 44% based onan unstretched portion of said monofilament and a soiling index of lessthan 10 after five soiling cycles.
 2. The soil-repellent andabrasion-resistant monofilament as defined in claim 1 and having afibrillation grade less than 0.1 at 2000 rpm.
 3. The soil-repellent andabrasion-resistant monofilament as defined in claim 1, wherein saidsilicon content is from 0.10 to 1.0% by weight.
 4. The soil-repellentand abrasion-resistant monofilament as defined in claim 3, wherein themonofilament shows substantially no fibrillation after being beatenagainst a solid body with a beat frequency of 2000 rpm for ten minutes.5. A soil-repellent and abrasion-resistant monofilament consisting of alinear silicone-modified polyethylene terephthalate having a siliconcontent of from 0.05 to 2.0% by weight based on the monofilament inwhich 0.15 to 5.0% by weight of a polydimethyl siloxane of formula I:

    --CO--R.sub.1 -- Si(R.sub.2).sub.2 --O!.sub.n --Si(R.sub.2).sub.2 --R.sub.1 --CO--                                                    (I),

wherein n is the number of repeated --Si(R₂)₂ --O-- groups, R₂ is amethyl group and R₁ represents a radical having 2 to 20 carbon atoms, isinserted into a polyethylene terephthalate polymer chain thereof;wherein said monofilament has a cross-section with a diameter of from0.1 to 1.0 mm, a homogeneous composition over said cross-section, atenacity of at least 36 cN/tex, an elongation of less than 44% based onan unstretched portion of said monofilament, a soiling index of lessthan 10 after five soiling cycles and a fibrillation grade less than 0.1at 2000 rpm.
 6. A process of making a soil-repellent andabrasion-resistant monofilament, said process comprising the steps of:a)transesterifying dimethylterephthalate with ethylene glycol to form areaction mixture; b) adding to said reaction mixture from 0.15 to 5percent by weight of a polydialkylsiloxane of formula I:

    --CO--R.sub.1 -- Si(R.sub.2).sub.2 --O!.sub.n --Si(R.sub.2).sub.2 --R.sub.1 --CO--                                                    (I),

wherein n is the number of repeated --Si(R₂)₂ --O-- groups and R₁ or R₂represent equal or different radicals with 2 to 20 carbon atoms; c)after the adding of step b), polycondensing to form a linearsilicone-modified polyethylene terephthalate in which said polydialkylsiloxane of formula I is inserted into a polyethylene terephthalatepolymer chain thereof; and d) extruding the linear silicone-modifiedpolyethylene terephthalate to form a monofilament consisting of thesilicone-modified polyethylene terephthalate with a silicon content offrom 0.05 to 2.0% by weight and having a cross-section with a diameterof from 0.1 to 1.0 mm, a homogeneous composition over saidcross-section, a tenacity of at least 36 cN/tex, an elongation of lessthan 44% based on an unstretched portion of said monofilament and asoiling index of less than 10 after five soiling cycles.
 7. The processas defined in claim 6, wherein the monofilament has a fibrillation gradeless than 0.1 at 2000 rpm.
 8. The process as defined in claim 6, whereinsaid silicon content is from 0.10 to 1.0% by weight.
 9. The process asdefined in claim 8, wherein the monofilament shows substantially nofibrillation after being beaten against a solid body with a beatfrequency of 2000 rpm for ten minutes.
 10. A process of making asoil-repellent and abrasion-resistant monofilament, said processcomprising the steps of:a) transesterifying dimethylterephthalate withethylene glycol to form a reaction mixture; b) adding to said reactionmixture from 0.15 to 5 percent by weight of a polydimethylsiloxane offormula I:

    --CO--R.sub.1 -- Si(R.sub.2).sub.2 --O!.sub.n --Si(R.sub.2).sub.2 --R.sub.1 --CO--                                                    (I),

wherein n is the number of repeated --Si(R₂)₂ --R₁ --CO-- groups, R₂ isa methyl group and R₁ represents a radical having 2 to 20 carbon atoms;c) after the adding of step b), polycondensing to form a linearsilicone-modified polyethylene terephthalate in which said polydimethylsiloxane of formula I is inserted into a polyethylene terephthalatepolymer chain thereof; and d) extruding the linear silicone-modifiedpolyethylene terephthalate to form a monofilament consisting of thesilicone-modified polyethylene terephthalate with a silicon content offrom 0.05 to 2.0% by weight and having a cross-section with a diameterof from 0.1 to 1.0 mm, a homogeneous composition over saidcross-section, a tenacity of at least 36 cN/tex, an elongation of lessthan 44% based on an unstretched portion of said monofilament, a soilingindex of less than 10 after five soiling cycles and a fibrillation gradeless than 0.1 at 2000 rpm.